Curveball

- Docs: use curl instead of wget, update figures and tables
- Force xlrd v1 because v2 does not support xlsx
- Bumped some dependency versions
- Fixed small minor bugs

- Bug fixes

- Removed central estimation in `curveball.models.*_ci` functions.
- Build with conda-build version 1; version 2 caused problems on travis-ci (see [PR 1342](https://github.com/conda/conda-build/pull/1342)).
- Removed python-dateutil dependency.

- Fixed bug: `find_K_ci` was called with wrong args in `cli.py`.
- Added `--nsamples` to CLI
- Test with CLI with `--ci`
- Adjusted to [seaborn API change](https://github.com/mwaskom/seaborn/commit/69e7f371d27725160d092a528c96cf1fce99b8b4): `sns.Grid`->`sns.axisgrid.Grid`

- added tests for `competitions.py`
- fixed bad docstrings
- avoid loading qt
- add `y0` arg to competition CI function
- update to lmfit 0.9.3 to solve bug
- confidence interval version for `fit_and_compete`
- fix bootstrap bug
- raise dev status to alpha in PyPI identifiers
- plot sampled curves and sampled model fits
- small bug fixes

See commit log for more details

- When using a `find_*_ci` function, it now also returns an estimate - the average of all samples (bug fix).
- Added `find_K_ci` to find the confidence interval of the K parameter (closes 131)